This invention relates to the preparation of a class of functionalized thermoplastic polymers known as ionomers. In one aspect, the invention relates to an improved process for neutralizing acid functionality of a polymeric material to prepare an ionomer. An ionomer is an ion containing polymer.
Ionomeric block copolymers are useful as toughening components in blends with harder but more brittle polymers. U.S. Pat. No. 4,797,447 discloses blends of functionalized block copolymers with polyesters. Copolymers disclosed as useful as toughening components include copolymers of monoalkenyl arenes such as styrene or alphamethylstyrene and conjugated dienes such as butadiene and isoprene.
Blending of elastomeric copolymers, which have not been functionalized, with more brittle polymers often leads to a product which demonstrates little improvement in toughness. The reasons for this are rather well understood and stem in part from the fact that most combinations of polymer pairs are not miscible, although a number of notable exceptions are known. More importantly, most polymers adhere poorly to one another as a result of incompatible structure or polarity. The more brittle polymers are often polar whereas the unfunctionalized elastomeric copolymers are generally non-polar. As a result, interfaces between component domains (a result of their immiscibility) represent areas of severe weakness in blends and, therefore, provide natural flaws and cracks which result in facile mechanical failure.
Elastomeric copolymers containing ionomeric blocks form stronger crosslinkages with some engineering thermoplastics which contain polar functionality than corresponding copolymers which contain acid or other types of functionality. Block copolymers containing ionomeric blocks therefore greatly improve performance as a toughener for these thermoplastics.
Ionomers are not only useful as blend components to toughen engineering thermoplastics which would be otherwise unacceptably brittle, but are also useful as coatings, lubricating oil viscosity index (VI) improvers and adhesives. As coatings, VI improvers and adhesives the ionomers may be useful as blocks within block copolymers, random copolymers, homopolymers or blends with other polymers.
Commercially available ionomers include "Surlyn" (the trade name of an ionomer manufactured and sold by DuPont Co., U.S.A.), "Copolene" (the trade name of an ionomer manufactured and sold by Asahi-Dow Co., Ltd., Japan), "Hi-Milan" (the trade name of an ionomer manufactured and sold by Mitsui Polychemical Co., Ltd., Japan) and Nafion (the trade name of an ionomer manufactured by DuPont Co., U.S.A.).
A wide variety of metal ions are in the prior known to be effective as cations for neutralization of the acid functionality of the modified polymer blocks. Monovalent, divalent, trivalent and quadravalent ions of metals of Groups IA, IB, IIA, IIB, IIIA, IVA, and VIII of the Periodic Table are used. For example, there may be mentioned ions such as Na+, Li+, K+, Cs+, Ag+, Ni+.sup.2, Mg.sup.2+, Ca.sup.2+, Ba.sup.2+, Zn.sup.2+, Sr.sup.2+, Hg.sup.2+, Fe.sup.2+, Al.sup.3+ and Fe.sup.3+. These ions are conventionally reacted with substrate polymers in the form of metal hydroxides, alcoholates, oxides and low carboxylic acid salts.
The reaction of the metal ion source and the acid groups typically takes place in a high temperature melt phase due to the insolubility of the metal ion sources in solvents which will also dissolve the functionalized polymers. When high levels of neutralization are required, the melt phase reaction is particularly undesirable. This is due to a large excess of metal ion source required because of the inefficiency of contact between the functionalized polymer and the metal ion source. The excess of reactants results in higher material costs, and contamination of the product with unreacted excess of metal ion source. Many prior art metal ion sources also result in by-products which contaminate the product. Contamination of product by excess reactants and reaction by-products causes higher haze, and could otherwise be detrimental to product properties.
It is therefore an object of this invention to provide an improved process for preparing ionomers. In one embodiment, it is an object of the invention to provide a process wherein the neutralization of polymer functional groups may be performed in a solution reaction medium. Hence, large excesses of the metal ion source are not required. It is a further object to provide a neutralization process wherein the by-products of the neutralization reaction are easily removed from the reaction medium.